Deep Drilling Small Diameter Hole

[Sailplane Driver]

I need to manually drill a 0.125 diameter hole 2 inches deep in 304 free machining stainless. I'm using a mini mill. I drilled with a center drill about .250 deep to make sure the drill started true. I then used a jobber length solid carbide drill, peck drilling roughly 0.100 per peck with cutting oil. I withdrew the drill every two or three pecks and added oil into the hole. The drill snapped at the top of the flutes at the same depth of the flutes. I kept RPM low, about 1500. Max machine RPM is a bit below 4000. What am I doing wrong?

 

[Eddyde]

Try a cobalt drill, carbide is very brittle. Also tighten the chuck in all three holes, it helps keep the drill concentric.

 

[RandyM]

Try a cobalt drill, carbide is very brittle. Also tighten the chuck in all three holes, it helps keep the drill concentric.

HHHmmm, first I have heard of this. Makes sense though.

 

[Eddyde]

HHHmmm, first I have heard of this. Makes sense though.

My mentor, an old school tool & die maker, taught me that way. I usually go around twice, snugging up first then the tightening round. Helps prevent slipping too. You can really feel the additional turn in the other holes.
I don't bother when grip or accuracy aren't a concern.

 

[Sailplane Driver]

My mentor, an old school tool & die maker, taught me that way. I usually go around twice, snugging up first then the tightening round. Helps prevent slipping too. You can really feel the additional turn in the other holes.
I don't bother when grip or accuracy aren't a concern.

For what it's worth, I do that virtually always. I find it gives better grip and is less prone to slippage compared to tightening one hole only.

 

[JimDawson]

Cobalt drill is good. I would spin maybe 1500-2000, and maybe 0.05 peck. But don't be too gentle with the feed, it will want to work harden on you.

 

[Robert LaLonde]

Yep, if you ain't cutting your rubbing, and rubbing is work hardening. 304 will definitely work harden.

Interestingly I too need to deep drill some .125 holes. I want to make some solid tool holders for small tools for my big mill. Since these are tool holders they need to be tight tolerance and straight. Not stainless though. Using 4140 HT. I haven't decided for sure how I am going to do it, yet, but I am thinking of finding or making a micro round edge carbide boring bar for the final size.

 

[mikey]

Cobalt drill is good. I would spin maybe 1500-2000, and maybe 0.05 peck. But don't be too gentle with the feed, it will want to work harden on you.

I agree. 304 will work harden readily so you have to keep the drill cutting. Personally, when I drill 304 I use as much pressure as the drill will allow and I do not dwell, ever. With small drills, as you get deeper the flutes will clog up with chips and the drill will stop cutting effectively. Pull the drill, clear the chips, lube and go again. If the work hardens and the drill stops cutting, increase pressure to get through the hard part.

 

[Sailplane Driver]

I agree. 304 will work harden readily so you have to keep the drill cutting. Personally, when I drill 304 I use as much pressure as the drill will allow and I do not dwell, ever. With small drills, as you get deeper the flutes will clog up with chips and the drill will stop cutting effectively. Pull the drill, clear the chips, lube and go again. If the work hardens and the drill stops cutting, increase pressure to get through the hard part.

Pretty much what I was doing until the drill broke. I tried to keep the chips clear but something grabbed. Do you think a constant mist coclling instead of manual oiling would help or should I stick with cutting oil?

 

[mikey]

Pretty much what I was doing until the drill broke. I tried to keep the chips clear but something grabbed. Do you think a constant mist coclling instead of manual oiling would help or should I stick with cutting oil?

I use cobalt drills for stainless and only use cutting oil. Haven't used a carbide drill or mist cooling so I don't know for sure. I'm with the other guys. I would change to a cobalt drill, slow the speed a bit and use as much pressure as needed to keep the drill cutting.